A device using, e.g., a NAND-type flash memory which is an electrically rewritable memory has been proposed as a nonvolatile semiconductor memory device. A NAND-type flash memory consists of a plurality of NAND cell units arranged like a matrix. A NAND cell unit consists of a memory string constituted by a plurality of series-connected nonvolatile memory cells, and of two select transistors connected to both ends of the memory string.
A technique using self-boosting is known as that of writing data to a nonvolatile memory cell of a NAND cell unit (see, e.g., JP-11-016381-A). It is recognized that the enhancement of the cut-off characteristic of a select transistor is required to perform self-boosting (see, e.g., JP-2001-284554-A). Further, a sidewall transfer process (or sidewall processing process) of forming a fine pattern exceeding the resolution limit of lithography is known as a technique for miniaturization of a nonvolatile semiconductor memory device (see, e.g., JP-08-055908-A).